This is just fancy sounding talk for the myriad of techniques in good cooking that create good flavor in dishes. It doesn't refer to any one specific thing.
Some examples of techniques used to develop flavor include:
- Seasoning (properly salting)
- Browning meats and other foods properly to bring out their flavor
- Creating pan sauces or other uses from fond to enhance the flavor of a dish
- Using herbs and spices skilfully to compliment the food
- Balancing flavors with acidic ingredients like lemon juice or tomatoes
- Cooking properly to allow the flavor of individual ingredients to shine through
- Blooming or toasting spices to bring out their more aromatic qualities
- Using alcohol to enhance certain flavors not soluble in water or fat
There are no doubt many, many more things that could be included in a list like this.
After all, it sounds far more professional to say "the flavor in this dish is not well developed" than to say "this dish is bad."